Comtech EF Data MM200 - Manual (Page 56)

User Interfaces MM200 High-Speed Microwave Modem
4-32 TM086 - Rev. 4.1
The format and structure of the COMMSPEC message exchanges are described herein. Decimal
numbers have no suffix; hexadecimal numbers end with a lower case h suffix and binary values
have a lower case b suffix. Thus, 22 = 16h = 000010110b. The principal elements of a data
frame, in order of occurrence, are summarized as follows:
- the message format header character, or ASCII sync character, that defines the
beginning of a message. The character value is always 16h.
- the Byte Count is the number of bytes in the field, ranging from 0
through TBD. This field is 2 bytes long for the MM200 protocol.
- the Source Identifier defines the message originator’s multidrop address. Note
that all nodes on a given control bus have a unique address that must be defined.
- The Destination Identifier specifies the multidrop address of the device(s)
to which the message is sent.
- The FSN is a tag with a value from 0 through 255 that is sent
with each message. It assures sequential information framing and correct equipment
acknowledgment and data transfers.
- The Operation Code field contains a number that identifies the message type
associated with the data that follows it. Acknowledgment and error codes are returned in this
field. This field is 2 Bytes for the MM200 protocol.
<...DATA...> - The Data field contains the binary, data bytes associated with the
. The number of data bytes in this field is indicated by the value.
- The checksum is the modulo 256 sum of all preceding message bytes,
excluding the character. The checksum determines the presence or absence of errors
within the message. In a message block with the following parameters, the checksum is
computed as shown below in Table 4-4.
Table 4-4. Checksum Calculation Example
BYTE FIELD DATA CONTENT RUNNING CHECKSUM
(Byte 1) 00h = 00000000b 00000000b
(Byte 2) 02h = 00000010b 00000010b
F0h = 11110000b 11110010b
2Ah = 00101010b 00011100b
09h = 00001001b 00100101b
(Byte 1) 00h = 00000000b 00101000b
(Byte 2) 03h = 00000011b 00101000b
(Byte 1) DFh = 11011111b 00000111b
(Byte 2)
FEh = 11111110b
00000101b
Thus, the checksum is 00000101b; which is 05h or 5 decimal. Alternative methods of
calculating the checksum for the same message frame are: